In recent years, as energy saving of apparatuses has advanced, attention has been given to nitride semiconductor light emitting devices including nitride semiconductor light emitting elements such as light emitting diodes using nitride semiconductors and semiconductor lasers. For example, a light source obtained by combining a nitride semiconductor light emitting device and fluorescent material, etc. can output white light at high power, and has features such as a small size, high efficiency, long life as compared to conventional incandescent lamps, fluorescent lamps, and high pressure mercury lamps. Therefore, the conventional lamps have been rapidly replaced with such light sources as light sources used for display devices such as projectors, liquid crystal backlights, etc.
On the other hand, nitride semiconductor light emitting elements having waveguides, typified by nitride semiconductor lasers, have such a feature that directivity indicating a light emitting direction is high. Thus, industrial application of nitride semiconductor light emitting devices including such nitride semiconductor light emitting elements has also advanced. For example, practical use of laser scribe devices and laser annealing devices using the nitride semiconductor light emitting devices as light sources has been considered. High light output at a watt class higher than or equal to one watt is required for light sources used for such devices. Therefore, the output of the nitride semiconductor light emitting device has also to be increased.
A light emitting element such as a nitride semiconductor light emitting element emits light through recombination of electrons and holes (electron-hole pairs) injected into an active layer formed by, for example, a quantum well of the light emitting device. At this time, energy which has not been converted to the light results in Joule heat, which increases the temperature of the element. In order to increase the output of the nitride semiconductor light emitting device, the amount of applied current (electrons, holes) has to be increased. However, an increase in current increases heat generated in the nitride semiconductor light emitting element. The heat generation increases the temperature of the element, which causes a phenomenon in which some of the electrons or the holes are not injected into the active layer, and overflow (a phenomenon called carrier overflow). In this case, light output is no longer increased even when the amount of the current is increased, and eventually, a phenomenon in which the light output is saturated (optical saturation) occurs. Thus, in order to obtain the high light output, it is required to reduce an increase in chip temperature and reduce the carrier overflow to efficiently convert the electron-hole pairs to light.
Therefore, to reduce such carrier overflow, a layer having a large energy band gap is inserted near an active layer in a conventional technique. International Patent Publication No. WO 2005/034301 (hereinafter referred to as Document 1) describes a configuration of a conventional nitride semiconductor laser. FIG. 17A is a view illustrating the configuration of the conventional nitride semiconductor laser. FIG. 17B illustrates part of an energy band gap of the conventional nitride semiconductor laser, the part being close to a conduction band.
As illustrated in FIG. 17A, the conventional configuration of the nitride semiconductor laser has a stacked structure obtained by stacking an n-AlGaN clad layer 1003, an InGaN active layer 1005, a p-AlGaN electron overflow suppressing layer 1010 (which is referred to as an electron block layer in the present specification), a p-AlGaN clad layer 1011, and a p-GaN contact layer 1012 on a GaN substrate 1001 and an n-GaN layer 1002.
The electron block layer 1010 is a layer provided to prevent electrons injected from the n-GaN layer 1002 from overflowing from the active layer 1005 into the p-AlGaN clad layer 1011. As illustrated in FIG. 17B, the electron block layer 1010 has a larger energy band gap than the layers in the periphery of the electron block layer 1010. In order to increase the energy band gap, an AlGaN layer having a high Al content is used as the electron block layer 1010.
On the other hand, a piezoelectric field may be induced in the nitride semiconductor due to lattice mismatch strain. The lattice constant of the above-described p-AlGaN electron block layer 1010 is smaller than the lattice constant of the GaN substrate 1001. Therefore, tensile strain is induced in the p-AlGaN electron block layer 1010, and the tensile strain causes the piezoelectric field. The piezoelectric field reduces the effect of confining electrons by the p-AlGaN electron block layer.
Japanese Unexamined Patent Publication No. 2005-217421 (hereinafter referred to as Document 2) describes that a piezoelectric field reduces the effective height of a barrier against electrons. Thus, Document 2 proposes increasing the thickness of a p-AlGaN block layer for confinement of the electrons. When the thickness of the AlGaN layer is increased, elastic lattice mismatch strain is reduced by dislocation or the occurrence of cracks. Thus, the piezoelectric field caused by the lattice mismatch strain is reduced, thereby increasing the height of the barrier against electrons of a conduction band.
However, the dislocation or plastic deformation such as the cracks significantly influences the characteristics and the reliability of the element. In particular, the AlGaN layer on the GaN substrate has the tensile strain, and thus cracks are likely to occur. In the periphery of the cracks, a surface of a wafer is not flat, so that fabrication of normal elements becomes difficult.